Electronic device based on electrowetting effect
An electronic device comprises an array of electro wetting cells, each cell comprising first and second liquids ( 14,18 ) which are immiscible in each other. An electrode arrangement ( 60 a ,60 b ) is used to control the position of the interface between the first and second liquids, and comprises a first electrode and a second electrode. Each cell comprises a first switching device ( 64 a ) for applying a cell drive voltage to the first electrode ( 60 a ), and a second switching device ( 64 b ) for simultaneously applying the same cell drive voltage to the second electrode ( 60 b ). The cell circuit can be produced without requiring any additional control lines to those required for a single electrode and associated switching device, but the use of two switching devices enables the ultimate signal applied to each electrode to be different as a result of different capacitive effects. This then enables control over the way the liquid interface moves in response to control voltages.
1. An electronic device comprising an array of cells, each cell comprising:
first and second liquids which are immiscible in each other;
a hydrophobic layer;
an electrode arrangement for applying a control voltage to the hydrophobic layer for controlling the position of the interface between the first and second liquids,
wherein the electrode arrangement comprises a first electrode and a second electrode, wherein each cell comprises a first switching device for applying a cell drive voltage to the first electrode, and a second switching device for simultaneously applying the same cell drive voltage to the second electrode, and wherein each cell further comprises a first storage capacitance between the first electrode and a capacitor electrode, and a second storage capacitance between the second electrode and the capacitor electrode.
2. The device as claimed in claim 1 , wherein the ratio of a parasitic capacitance of the first switching device to the first storage capacitance is different to the ratio of the corresponding parasitic capacitance of the second switching device ( 64 b ) to the second storage capacitance.
3. The device as claimed in claim 2 , wherein each switching device comprises a thin film transistor, and the parasitic capacitance comprises the source-drain capacitance (Cgs).
4. The device as claimed in claim 3 , wherein each transistor has substantially the same source-drain capacitance (Cgs) and the first storage capacitance is different to the second storage capacitance.
5. The device as claimed in claim 1 , wherein the first electrode has a first area and the second electrode has a second, greater area.
6. The device as claimed in claim 1 , wherein the cell has a substantially quadrilateral shape, bounded by cell walls, and wherein the first electrode comprises an outer electrode which borders three of the four sides of the quadrilateral, and the second electrode comprises an inner electrode which borders two adjacent sides of the quadrilateral at a corner location.
7. The device as claimed in claim 6 , wherein the second electrode occupies a central region of the cell.
8. The device as claimed in claim 1 , wherein the first and second electrodes are co-planar.
9. The device as claimed in claim 1 , wherein the first liquid is water based and the second liquid is oil based.
10. The device as claimed in claim 1 , comprising an array of rows and columns of cells, with row and column addressing conductors, wherein the switching devices of each cell are both connected to and switched by the same respective row conductor, and wherein the switching devices of each cell are both connected to the same respective column conductor for switching the column conductor voltage to the respective electrode of the electrode arrangement.
11. The device as claimed in claim 1 , wherein a kickback voltage (Vkb 1 ,Vkb 2 ), introduced at the end of the addressing using the switching devices, is different for the two switching devices of each cell.
12. The device as claimed in claim 1 , wherein each electrode is provided on the opposite side of the hydrophobic layer to the first and second liquids of the cell.
13. The device as claimed in claim 1 , comprising a display device, wherein each cell comprises a display pixel.
14. The device as claimed in claim 13 , wherein each cell comprises a clear liquid and a coloured liquid, the coloured liquid providing subtractive colour filtering.
15. The device as claimed in claim 14 , comprising a stack of three display devices, one having cells with a magenta coloured liquid, one having cells with a cyan coloured liquid and one having cells with a yellow coloured liquid.
16. A method of operating an electronic device comprising an array of cells, each cell comprising first and second liquids which are immiscible in each other and a hydrophobic layer, the method comprising, for each cell:
applying a cell drive voltage to the hydrophobic layer for controlling the position of the interface between the first and second liquids using first and second electrodes, the same drive voltage being applied to the first and second electrodes simultaneously through respective switching devices, wherein each cell comprises a display pixel, and
causing a kickback voltage to be introduced at the end of addressing using the switching devices, wherein the kickback voltage is different for the two switching devices of each cell.